The present invention relates to a method of determining the transition impedance between two parts of a subdivided neutral electrode used in high-frequency surgery. In addition, a measurement apparatus for determining the transition impedance between two parts of a subdivided neutral electrode used in high-frequency surgery is described, which comprises a resonant circuit into which the two electrode parts can be incorporated in parallel as well as a current-supply device by means of which an alternating measurement voltage can be impressed into the resonant circuit.
In high-frequency surgery high-frequency electrical energy is supplied to the tissue to be treated. A distinction is generally made between a monopolar and a bipolar mode of employing the high-frequency current.
For monopolar use only one active electrode is provided, to which a high-frequency alternating voltage is supplied. Furthermore, it is necessary to apply a neutral electrode to a large area of the patient's body, by means of which the circuit is closed owing to current flow through tissue between the active and neutral electrodes. The shape of the active electrode depends on the particular purpose for which it is employed. The surface area of the active electrode by way of which alternating current is conducted into the tissue is relatively small, so that in the immediate surroundings of the active electrode there is a high current density and hence also a large amount of heat is developed.
The current density decreases rapidly at progressively greater distances from the active electrode, insofar as considerable differences in tissue conductivity do not increase the current density at other places in the body. The alternating currents supplied to the active electrode are conducted away through the neutral electrode. Accordingly, care should be taken to place the neutral electrode in contact with the patient's body over a large area, so that it presents only a slight transition resistance to the high-frequency alternating current.
For bipolar use two active electrodes are provided, between which the tissue to be treated is enclosed. The electrical circuit is closed by the tissue lying between the two active electrodes, which thus becomes heated when a high-frequency alternating voltage is applied. During this process the major proportion of the current flows between the two active electrodes, but in case of aberration there may be some current flow in neighboring parts of the patient's body. In order to conduct such diverted currents away from the body over a large area, so that they do not cause any undesired burns, even when bipolar instruments are used in some cases a neutral electrode is employed, which again should be placed in contact with the patient's body over a large area. The neutral electrode prevents an elevated current density in parts of the body other than that between the active electrodes, and avoids undesired burns.
If part of the neutral electrode should become separated from the tissue, so that the current flow is restricted to those parts of the neutral electrode that remain in contact, higher current densities and hence burning of the tissue can result. As is shown by the following references to the state of the art, various monitoring systems are known by means of which the transition resistance of the neutral electrode can be evaluated.
A high-frequency surgical appliance with a neutral electrode divided into two parts and circuitry for measuring the resistance between the two parts of the neutral electrode is described in the document DE-AS 1 139 927. In this appliance there is provided an accessory direct-current circuit with a source of direct current, to which the two electrode parts are connected in series, so that the only connection between the two electrode parts consists of the patient's body. If the ohmic resistance measured between the two electrode parts exceeds a certain limiting value, the high-frequency generator of the surgical appliance is switched off and/or an alarm system is triggered.
A control circuit for controlling the output of a high-frequency generator in dependence on the measured high-frequency current flowing between an active electrode and a neutral electrode is known from U.S. Pat. No. 3,913,583. The high-frequency current intensity depends on the apparent resistance between active electrode and neutral electrode, so that the output of the high-frequency generator is ultimately regulated in dependence on the apparent resistance.
Furthermore, from U.S. Pat. Nos. 3,933,157, 5,087,257 and WO 9619152 neutral-electrode monitoring systems are known by means of which the apposition of a two-part neutral electrode to a patient is evaluated by measuring the apparent resistance between the two parts of the neutral electrode. For this purpose a measurement circuit is provided into which the two electrode parts are incorporated in such a way that the measurement circuit is closed by way of the part of the patient's tissue that is situated between the two electrode parts. To determine the apparent resistance, an alternating voltage is applied to the measurement circuit.
From U.S. Pat. No. 4,200,104 a monitoring system for a two-part neutral electrode is known that is intended to detect a separation of part of the neutral electrode from the patient by measuring the capacitance between the two parts of the neutral electrode. The proposed circuitry for capacitance measurement comprises a monostable multivibrator to the input of which is delivered a signal at constant frequency. The two electrode parts of the neutral electrode are incorporated into the multivibrator circuitry in such a way that the capacitance between these two electrode parts influences the pulse width of the output signal from the monostable multivibrator. The capacitance is to be found by evaluating this pulse width, and this information is then used to draw conclusions about the area over which the neutral electrode is in contact with the patient. However, the pulse width of the multivibrator circuit is also influenced by the ohmic resistance between the two electrode parts, which can also change depending on the contact area of the neutral electrode. Separation of the influences exerted by the capacitance from those exerted by the ohmic resistance seems to be impossible with the circuitry proposed here.
DE 197 14 972 A1 also describes an apparatus for monitoring the application of a bipartite neutral electrode. This apparatus comprises an impedance sensor that detects the transition resistances at the surface of each of the two electrode parts connected in series. The apparatus is galvanically separated from the electrode-part surfaces by means of a transformer. Measurement of the transition resistance at the patient makes use of the fact that this resistance is in each case disposed parallel to the transformer and to a capacitance, and accordingly a parallel oscillating circuit is formed. By triggering the damped oscillating circuit at its resonant frequency, the voltage is determined solely by the transition resistance at the patient. Accordingly, the voltage measured at the resonant frequency allows conclusions to be drawn about the transition resistance at the patient.
The transition between the body of the patient and the neutral electrode opposes the high-frequency alternating current not only by an ohmic resistance, but also by a capacitive resistance determined by charging effects. The above-mentioned monitoring systems in the state of the art are limited to detecting either the ohmic resistance or the apparent resistance in this transition impedance.
The document U.S. Pat. No. 4,200,104 also describes a device for measuring capacitance and thereby estimating the contact area of the neutral electrode, but the capacitance measurement by the measurement device proposed there is influenced by the ohmic component of the transition impedance.